The invention relates to a cellular communications system comprising mobile communication units.
Communication systems are known in which the users of the system have mobile terminals. These terminals can communicate with each other through the infrastructure of a xe2x80x98mobile networkxe2x80x99. This network includes base stations, which are central radio transmitter- and receiver units. The base stations are linked to one another by the network. A terminal can also communicate through the mobile network to a traditional fixed line telephone network. A packet radio system may work in this way.
A terminal establishes a communication link to a base station via a radio signal, and communicates over this link. A terminal normally communicates over a radio link set up to the base station which provides the strongest signal, this usually being the base station which is nearest to the mobile terminal at that point in time.
Typically a communication pathway within the system outlined above consists of a first bi-directional radio link between a first mobile terminal and a nearby first base-station. From the first base station the communication link may typically further connect through a fixed line system to another, second base station. The second base station will communicate over a second bi-directional radio link with a second mobile terminal. Thus two-way communication between the mobile terminals is possible.
Communication systems such as that outlined above operate in accordance with internationally agreed standards. A recent standard for such systems is the European Telecommunications Standards Institute""s (ETSI) GSM standard number 05.02. Version 6.3.0 of this standard has recently been made available. This standard relates to communication networks and mobile terminals which may communicate using either circuit switched (CS) communication links and/or group switched packet (GPRS) communication links.
A communication in accordance with ETSI standard GSM 05.02 permits the exchange of voice communication and/or data between mobile terminals. Furthermore, this communication system allows mobile terminals of varying sophistication to operate within it, and also foresees the mobile network being able to operate in up to three different modes. These network modes are referred to as xe2x80x98Network mode 1xe2x80x99, xe2x80x98Network mode 2xe2x80x99 and xe2x80x98Network mode 3xe2x80x99 respectively.
The three xe2x80x98network modesxe2x80x99 in which the network can operate have some similarities, but differ in several aspects. For the purposes of the invention, it is important to consider how a base station sets up a communication link with a mobile terminal in each of the three different modes.
In all three network modes, a communication link initiated by the mobile network is set up by the base station broadcasting a signal which the mobile terminal receives. The signal used to set up a communication link is referred to as a xe2x80x98pagingxe2x80x99 signal. When a mobile terminal receives and recognises a paging signal destined for it, the mobile terminal knows that it is to set up a communication link with the base station. For the mobile terminal to receive a paging signal, the terminal must know in advance when to turn on its radio receiver to xe2x80x98listenxe2x80x99 for a paging signal destined for that terminal. The mobile terminal is said to be in xe2x80x98idle modexe2x80x99 when it is waiting to be paged by the network.
The mobile terminals used in a communication system built in accordance with the standard GSM 05.02 may have one of three levels of capability. These are as follows:
A Class A terminal is capable of communicating with a base station of the mobile network using both circuit switched and GPRS links simultaneously. This means that the user of the class A terminal can, for example, speak to another user over a circuit switched communication link and simultaneously transmit a data file over a GPRS link.
A Class B terminal is only capable of communicating with a base station either by a circuit switched link or by a GPRS link at any one time. This means that the user of the class B terminal cannot speak to another user over a circuit switched communication link and simultaneously transmit a data file over a GPRS link.
When a class B terminal is in idle mode, the network can page the mobile terminal for either circuit switched or GPRS communication.
A Class C terminal is only capable of communicating with a base station by a GPRS link.
A further type of mobile terminal may also be present in the communication system. These are mobile terminals which can only receive CCCH channels, and which have no capability of receiving GPRS signals.
Next, it is important to understand how the network transmits a paging signal to a mobile terminal in accordance with ETSI GSM standard 05.02.
The mobile network can page a mobile terminal on one of two types of channel. The base stations of the mobile network may broadcast a signal on a control channel referred to as the Common Control Channel or xe2x80x98CCCHxe2x80x99 channel. A CCCH channel is the standard control channel used in prior art GSM networks to set up a communication link between the base station and a mobile terminal. However, the base stations may broadcast a signal on a control channel referred to as the Packet Common Control Channel or xe2x80x98PCCCHxe2x80x99 channel. The PCCCH channel is a physical channel which has been newly created for GPRS networks. The PCCCH channel can serve many purposes, of which paging of GPRS terminals is just one. Notably, it is not mandatory for a GPRS cell to have PCCCH channels. If a cell does not have PCCCH channels, then the base station in the cell pages a GPRS terminal on the xe2x80x98CCCHxe2x80x99 channel
FIG. 1 illustrates four CCCH channels in accordance with the GSM 05.02 standard. These channels are labelled CCCH(0)-CCCH(3).
The definition of a physical channel in GSM is that it comprises a frequency, which may change value or xe2x80x98hopxe2x80x99, and a time slot. Each of the four channels shown in FIG. 1 therefore represents a sequence of transmitted signals which may in reality be transmitted at different frequencies.
The number of circuit switched paging blocks available on each control channel (CCCH) is defined as N. In FIG. 1, N=512.
The CCCH signal is made up of a number of multiframes. In the arrangement of FIG. 1, this is 64. Each multiframe contains eight paging blocks, making the total of N=512 blocks.
Each block in FIG. 1 is shown to comprise 4 time-slots. The signals in the four time slots in any one block together make up the circuit switched paging block. Although shown consecutively in FIG. 1, the four time slots of a paging block are in reality likely to be broadcast in four identically numbered time slots in four consecutive groups of eight time slots. In each such group of eight time slots, the other seven time slots in the group do not relate to this paging block. Each of these groups of eight time slots is in fact a frame.
The use of the control channels according to the prior art arrangement of GSM 05.02 standard now needs to be considered. In accordance with this prior art arrangement, the mobile terminals in a cell are assigned paging blocks consecutively. The first 512 mobile stations can each be assigned a paging block on channel CCCH(0), because it can contain 512 blocks. See FIG. 1. If there are, for example, 1000 mobile terminals, then 512 terminals will be assigned paging blocks in channel CCCH(0), a further 488 terminals will be assigned paging blocks in channel CCCH(1), and the channels CCCH(2) and CCCH(3) will not transmit paging signals to any mobile terminals, so will remain unused.
In practice, both the network and a particular mobile terminal are able to calculate the individual paging block which the base station will use to make contact with that particular mobile terminal. Thus the mobile terminal will be able to monitor for a paging signal at the correct time point to receive any paging signal broadcast to it.
In the GSM system, there are four CCCH channels which are all broadcast on one frequency. This frequency changes from time to time, in other words it xe2x80x98hopsxe2x80x99. There may be several PCCCH channels in a cell. The PCCCH channels are each broadcast on a different frequency.
The three Network modes specified in the GSM 05.02 standard are as follows:
In this mode, the paging of mobile terminals is coordinated by the network. All circuit switched paging for class B terminals is re-routed by the network into blocks which are reserved for GPRS paging blocks on the PCCCH channel.
When the network is operating in network mode 1, any class B mobile terminal only needs to monitor the GPRS paging block positions in order to detect any paging signal destined for that terminal.
If a particular cell of the communication system has a base station which can broadcast a PCCCH channel, then the base station broadcasts all the circuit switched and GPRS the paging blocks on the PCCCH channel when the network is in network mode 1.
In this mode, the cell does not broadcast a PCCCH channel. There is no co-ordination of paging signals at the network level.
The operation of a mobile terminal when the network is in network mode 2 can take one of several forms.
The mobile terminal may support a mode of operation called xe2x80x98SPLIT_PG_CYCLExe2x80x99. xe2x80x98SPLIT_PG_CYCLExe2x80x99 is a GPRS parameter which is used to know the position of the paging signals within the signal broadcast from the base station. If the mobile terminal supports SPLIT_PG_CYCLE then, when in idle mode, a class B mobile terminal has to monitor for CS paging signals and for GPRS paging signals in two different positions on the CCCH channel. This monitoring on different positions has implications for the battery life of the mobile terminal, since the terminal needs to actively monitor for paging signals for a greater proportion of the time.
However, a mobile terminal may degrade to a state where it does not support SPLIT_PG_CYCLE. The mobile enters this state when the network is not broadcasting the PCCCH channel. In this state, the terminal monitors for CS and GPRS paging signals on CS positions.
In network mode 3, there are usually 2 channels, one CCCH and one PCCCH. However, there is no co-ordination at the network level. A class B terminal has to monitor for both CS paging on the CCCH channel and for GPRS paging on the PCCCH channel. Thus once again in this case, the mobile must spend a greater proportion of the time monitoring for a paging signal than would be the case when the network is operating in network mode 1.
When the network is operating in network mode 3, some cells may in fact operate without using PCCCH channels. A terminal does not therefore know what channels a cell can provide when the terminal enters the cell.
Finally, it is important to consider how a particular mobile terminal monitors for paging signals when it is in idle mode.
A GPRS terminal in idle mode only monitors one PCCCH channel. The terminal calculates which channel this is based on its own International Mobile Subscriber Identity (IMSI) number. This calculation in fact allows the mobile terminal to select one PCCCH channel, referred to as that mobile terminal""s xe2x80x98PCCCH_GROUPxe2x80x99, from the list of PCCCH channels. The PCCCH_GROUP is simply an index in the list of PCCCH channels.
Consider now that the GPRS mobile terminal knows which PCCCH channel to monitor for paging signals. The mobile terminal is further able to calculate the time or times at which it needs to monitor for paging signals on this PCCCH channel. The mobile terminal does this by calculating a variable termed the xe2x80x98PAGING_GROUPxe2x80x99. The PAGING_GROUP comprises the set of paging blocks, within the repeating cycle of 64 multiframes in GPRS, on which the network may broadcast a paging signal to the mobile terminal in question. There may just be one paging block in the PAGING_GROUP. However, in GPRS the mobile terminal may be paged on more than one of the paging blocks within the 64 multiframes, and in such a case the PAGING_GROUP identifies each of these blocks. A particular mobile terminal needs to monitor the PCCCH signal broadcast by the network at the times at which the base station broadcasts the paging blocks indicated in the PAGING_GROUP. This monitoring activity determines the amount of power which the mobile terminal""s receiver circuitry consumes, and hence influences heavily the power consumption of the mobile terminal and its battery life.
Any reduction in the amount of monitoring activity which a mobile terminal must carry out is highly desirable, since this will increase the battery life of the mobile terminal.
Analogously to the variables PCCCH _GROUP and PAGING_GROUP explained above, the variables CCCH_GROUP and PAGING_GROUP are defined for circuit switched mobile terminals.